The present invention relates generally to the field of radio communications systems, and more specifically to an inventive method to quickly determine the presence of a multi-carrier signal on a received channel using spectral characteristics of the received signal.
Multiple carriers are used in the transmission of digital signals to maximize correct reception of those signals in the presence of noise and interference and to maximize the data capacity of the transmission channel. An example of a multiple-carrier system is an orthogonal frequency division multiplexing (OFDM) system, which is used in both cellular telephone systems and in digital radio systems such as AM/FM in-band on-channel (IBOC) systems. Today's broadcast radio systems is comprised of channels that may contain multiple-carrier signals, such as IBOC channels, as well as the traditional analog channels that do not transmit multiple carrier signals, such as AM/FM channels.
Fast detection or determination of the presence of the multiple-carrier is needed to allow the receiver or the receiver's user to determine whether to continue to receive a channel if the multiple carriers are not present. For example, in an IBOC radio receiver, a particular tuned channel may be transmitting either traditional analog signals (i.e., non-IBOC signals) in which no multiple carriers present or IBOC signals in which multiple carriers are present. The user may prefer to listen only to IBOC channels and so not to continue receiving a channel if it does not transmit IBOC signals.
Before a digital audio receiver can recover the digital content of a multiple-carrier signal, the receiver must typically “lock” onto the digital signal. “Locking” means that the receiver adaptively configures itself through time, phase, and/or amplitude alignment mechanisms, such as by means of a phase-lock-loop, to receive the multiple-carrier signal in such a way that the digital content of the signal can be recovered. Receivers designed to receive complex digital modulation signals, such as IBOC signals, typically require a significant amount of time to lock onto the multiple carrier signal once the receiver is tuned to a channel where an IBOC signal is present.
The locking event or the lack of locking can be used as an indicator of the presence of the multiple-carrier signal. However, the long time period that is required for locking to occur, or to determine that no locking has taken place, conflicts with the general requirement for fast detection. Therefore, a different method that quickly detects or determines the presence of multiple-carrier signal components is needed.